Lithographic apparatus and device manufacturing method

ABSTRACT

A level sensor configured to measure a height level of a substrate arranged in a measurement position is disclosed. The level sensor comprises a projection unit to project multiple measurement beams on multiple measurement locations on the substrate, a detection unit to receive the measurement beams after reflection on the substrate, and a processing unit to calculate a height level on the basis of the reflected measurement beams received by the detection unit, wherein the projection unit and the detection unit are arranged next to the substrate, when the substrate is arranged in the measurement position.

This application claims priority and benefit under 35 U.S.C. §119(e) toU.S. Provisional Patent Application No. 61/313,285, filed Mar. 12, 2010.The contents of the foregoing application is incorporated herein in itsentirety by reference.

FIELD

The present invention relates to a level sensor, a lithographicapparatus and a method for determining a height map of a substrate foruse in a lithographic process.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In such a case, a patterning device, which isalternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.including part of, one, or several dies) on a substrate (e.g. a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned. Conventional lithographicapparatus include so-called steppers, in which each target portion isirradiated by exposing an entire pattern onto the target portion atonce, and so-called scanners, in which each target portion is irradiatedby scanning the pattern through a radiation beam in a given direction(the “scanning”-direction) while synchronously scanning the substrateparallel or anti-parallel to this direction. It is also possible totransfer the pattern from the patterning device to the substrate byimprinting the pattern onto the substrate.

The surface of a substrate on which a pattern should be projected isusually not completely flat. Moreover, a substrate can show thicknessvariation of several microns. This flatness and/or thickness variationof the substrate surface may result in incorrect projection of thepattern, for instance due to focus errors or imaging errors.

To correct for unflatness and/or thickness variation of a substrate, ithas been proposed to provide a level sensor, desirably integrated in thelithographic apparatus. Such a level sensor may be used to determine aheight map of a substrate, before a pattern is transferred, e.g.,projected, on the substrate. This height map may subsequently be used tocorrect the position of the substrate during transfer of the pattern onthe substrate.

SUMMARY

There is a desire to continuously increase throughput of a lithographicapparatus. Therefore, it is desirable to use less time for determining aheight map of a substrate.

The time involved in determining a height map using a level sensor maybe decreased by using multiple measurement locations on a substrate. Insuch a level sensor, a projection unit is provided to project, desirablysimultaneously, multiple measurement beams on multiple measurementlocations of the substrate.

In many lithographic apparatuses, the space available for the levelsensor is relatively small, or the space available for the level sensoris located at less favorable locations within the lithographicapparatus.

A potential drawback of a level sensor having multiple measurement beamsis that such a level sensor relatively may consume more space.

It is desirable, for example, to provide a level sensor for use in alithographic apparatus which level sensor can be more easily arranged inthe space available within a lithographic apparatus.

According to an embodiment of the invention, there is provided a levelsensor configured to measure a height level of a substrate arranged in ameasurement position, the level sensor comprising:

a projection unit to project multiple measurement beams on multiplemeasurement locations on the substrate,

a detection unit to receive the measurement beams after reflection onthe substrate, and

a processing unit to calculate a height level on the basis of thereflected measurement beams received by the detection unit,

wherein the projection unit and the detection unit are arranged next tothe substrate, when the substrate is arranged in the measurementposition.

According to an embodiment of the invention, there is provided alithographic apparatus comprising:

an illumination system configured to condition a radiation beam;

a support constructed to support a patterning device, the patterningdevice being capable of imparting the radiation beam with a pattern inits cross-section to form a patterned radiation beam;

a substrate table constructed to hold a substrate;

a projection system configured to project the patterned radiation beamonto a target portion of the substrate; and

a level sensor configured to measure a height level of a substrate onthe substrate table arranged in a measurement position, comprising:

-   -   a projection unit to project multiple measurement beams on        multiple measurement locations on the substrate,    -   a detection unit to receive the measurement beam after        reflection on the substrate, and    -   a processing unit to calculate a height level on the basis of        the reflected measurement beam received by the detection unit,

wherein the projection unit and the detection unit are arranged next tothe substrate, when the substrate is arranged in the measurementposition.

According to an embodiment of the invention, there is provided a methodto determine a height map of a substrate, the method comprising:

measuring a height of a substrate at a measurement location byprojecting a measurement beam on the substrate using a projection unit,receiving the reflected beam from the substrate using a detection unit,and determining a height level based on the reflected beam received bythe detection unit, wherein the projection unit and the detection unitare arranged next to the substrate, when the substrate is arranged in ameasurement position;

repeating the measuring for multiple measurement locations on thesubstrate; and

determining a height map of the substrate on the basis of the heightlevels.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic apparatus according to an embodiment ofthe invention;

FIG. 2 depicts a side view of a level sensor according to an embodimentof the invention;

FIG. 3 depicts a schematic view of a projection grating (left side) andresulting image (right side);

FIG. 4 depicts a side view of an embodiment of a detection grating and adetail of the detection grating;

FIG. 5 depicts a top view of an embodiment of a level sensor havingmultiple measurement locations;

FIG. 6 depicts a side view of an embodiment of a level sensor comprisinga tilt measuring device;

FIG. 7 depicts a top view of an embodiment of a beam delivery system fora multiple measurement location level sensor; and

FIGS. 8, 9 and 10 depict a top view, a cross section and a side view ofa further embodiment of a beam delivery system for a multiplemeasurement location level sensor.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic apparatus according to oneembodiment of the invention. The apparatus includes an illuminationsystem (illuminator) IL configured to condition a radiation beam B (e.g.UV radiation or any other suitable radiation), a patterning devicesupport structure (e.g. a mask table) MT constructed to support apatterning device (e.g. a mask) MA and connected to a first positioningdevice PM configured to accurately position the patterning device inaccordance with certain parameters. The apparatus also includes asubstrate support (e.g. a wafer table) WT constructed to hold asubstrate (e.g. a resist-coated wafer) W and connected to a secondpositioning device PW configured to accurately position the substrate inaccordance with certain parameters. The apparatus further includes aprojection system (e.g. a refractive projection lens system) PSconfigured to project a pattern imparted to the radiation beam B bypatterning device MA onto a target portion C (e.g. including one or moredies) of the substrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The patterning device support structure holds the patterning device in amanner that depends on the orientation of the patterning device, thedesign of the lithographic apparatus, and other conditions, such as forexample whether or not the patterning device is held in a vacuumenvironment. The patterning device support structure can use mechanical,vacuum, electrostatic or other clamping techniques to hold thepatterning device. The patterning device support structure may be aframe or a table, for example, which may be fixed or movable asrequired. The patterning device support structure may ensure that thepatterning device is at a desired position, for example with respect tothe projection system. Any use of the terms “reticle” or “mask” hereinmay be considered synonymous with the more general term “patterningdevice.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section so as to create a pattern in atarget portion of the substrate or any device to imprint a pattern intoimprintable medium. It should be noted that the pattern imparted to theradiation beam may not exactly correspond to the desired pattern in thetarget portion of the substrate, for example if the pattern includesphase-shifting features or so called assist features. Generally, thepattern imparted to the radiation beam will correspond to a particularfunctional layer in a device being created in the target portion, suchas an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam which is reflected by the mirrormatrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system”.

As here depicted, the apparatus is of a transmissive type (e.g.employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g. employing a programmable mirror array of a type asreferred to above, or employing a reflective mask).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate supports (and/or two or more patterning device supports).In such “multiple stage” machines the additional supports may be used inparallel, or preparatory steps may be carried out on one or moresupports while one or more other supports are being used for exposure.

The lithographic apparatus may also be of a type wherein at least aportion of the substrate may be covered by a liquid having a relativelyhigh refractive index, e.g. water, so as to fill a space between theprojection system and the substrate. An immersion liquid may also beapplied to other spaces in the lithographic apparatus, for example,between the mask and the projection system. Immersion techniques can beused to increase the numerical aperture of projection systems. The term“immersion” as used herein does not mean that a structure, such as asubstrate, must be submerged in liquid, but rather only means that aliquid is located between the projection system and the substrate duringexposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDincluding, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD if required, may be referred to as a radiation system.

The illuminator IL may include an adjuster AD configured to adjust theangular intensity distribution of the radiation beam. Generally, atleast the outer and/or inner radial extent (commonly referred to asσ-outer and σ-inner, respectively) of the intensity distribution in apupil plane of the illuminator can be adjusted. In addition, theilluminator IL may include various other components, such as anintegrator IN and a condenser CO. The illuminator may be used tocondition the radiation beam, to have a desired uniformity and intensitydistribution in its cross-section.

The radiation beam B is incident on the patterning device (e.g., mask)MA, which is held on the patterning device support structure (e.g., masktable) MT, and is patterned by the patterning device. Having traversedthe patterning device MA, the radiation beam B passes through theprojection system PS, which focuses the beam onto a target portion C ofthe substrate W. With the aid of the second positioning device PW andposition sensor IF (e.g. an interferometric device, linear encoder orcapacitive sensor), the substrate table WT can be moved accurately, e.g.so as to position different target portions C in the path of theradiation beam B. Similarly, the first positioning device PM and anotherposition sensor (which is not explicitly depicted in FIG. 1) can be usedto accurately position the patterning device MA with respect to the pathof the radiation beam B, e.g. after mechanical retrieval from a masklibrary, or during a scan. In general, movement of the patterning devicetable MT may be realized with the aid of a long-stroke module (coarsepositioning) and a short-stroke module (fine positioning), which formpart of the first positioning device PM. Similarly, movement of thesubstrate support WT may be realized using a long-stroke module and ashort-stroke module, which form part of the second positioner PW. In thecase of a stepper (as opposed to a scanner) the patterning device tableMT may be connected to a short-stroke actuator only, or may be fixed.Patterning device MA and substrate W may be aligned using patterningdevice alignment marks M1, M2 and substrate alignment marks P1, P2.Although the substrate alignment marks as illustrated occupy dedicatedtarget portions, they may be located in spaces between target portions(these are known as scribe-lane alignment marks). Similarly, insituations in which more than one die is provided on the patterningdevice MA, the patterning device alignment marks may be located betweenthe dies.

The depicted apparatus could be used in at least one of the followingmodes:

1. In step mode, the patterning device support MT and the substratesupport WT are kept essentially stationary, while an entire patternimparted to the radiation beam is projected onto a target portion C atone time (i.e. a single static exposure). The substrate support WT isthen shifted in the X and/or Y direction so that a different targetportion C can be exposed. In step mode, the maximum size of the exposurefield limits the size of the target portion C imaged in a single staticexposure.2. In scan mode, the patterning device support MT and the substratesupport WT are scanned synchronously while a pattern imparted to theradiation beam is projected onto a target portion C (i.e. a singledynamic exposure). The velocity and direction of the substrate supportWT relative to the patterning device support MT may be determined by the(de-)magnification and image reversal characteristics of the projectionsystem PS. In scan mode, the maximum size of the exposure field limitsthe width (in the non-scanning direction) of the target portion in asingle dynamic exposure, whereas the length of the scanning motiondetermines the height (in the scanning direction) of the target portion.3. In another mode, the patterning device support MT is kept essentiallystationary holding a programmable patterning device, and the substratesupport WT is moved or scanned while a pattern imparted to the radiationbeam is projected onto a target portion C. In this mode, generally apulsed radiation source is employed and the programmable patterningdevice is updated as required after each movement of the substratesupport WT or in between successive radiation pulses during a scan. Thismode of operation can be readily applied to maskless lithography thatutilizes programmable patterning device, such as a programmable mirrorarray of a type as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

In FIG. 1 the possible location of a level sensor 1 according to anembodiment of the invention in a lithographic apparatus is shown. Thesubstrate support WT and substrate W supported thereon are shown indashed lines in a measurement location of the substrate. In thismeasurement location a height level of the substrate W may bedetermined.

Level Sensor

FIG. 2 shows a level sensor generally indicated by reference numeral 1.The level sensor 1 is configured to determine a height map of asubstrate 2. This height map may be used to correct the position of asubstrate during projection of a pattern on the substrate 2. The levelsensor may be arranged in a stand-alone device, but is desirablyintegrated in a lithographic apparatus as shown in FIG. 1.

The level sensor 1 comprises a projection unit 3, a detection unit 4,and a processing unit 5. The projection unit 3 comprises a radiationoutput 6 (e.g., a radiation source or an outlet connected to a radiationsource elsewhere) and a projection grating 7. The radiation output 6 maybe, or connected to, any suitable radiation source. Desirably, it is, oris connected to, a broadband light source but a polarized ornon-polarized laser beam can also be used. The radiation output 6provides a measurement beam which is directed to the projection grating7.

The projection grating 7 comprises a periodic grating, i.e. a patternhaving a periodic structure resulting in a measurement beam having aperiodic structure in radiation intensity. The left side of FIG. 3 showsan example of such a projection grating having a periodic structure. Themeasurement beam with periodic radiation intensity is directed towards ameasurement location 8 via an optical reflector 9. The substrate islocated at this measurement location 8. Further optical elements may beprovided to guide the measurement beam towards the substrate 2. At themeasurement location 8 the measurement beam is reflected on thesubstrate 2 and runs via a second optical reflector 10, and possiblyfurther optical elements to the detection unit 4. The detection unit 4comprises a detection grating 11 and three detectors 12 a, 12 b, 12 c.

The detection grating 11 comprises a periodic structure as shown in FIG.4. This periodic structure comprises for each period an array of threesegments 13 a, 13 b, 13 c. The top surface of each segment within thearray of segments has a different angle with respect to the angle ofincidence of the measurement beam 14. As a result, the measurement beamis split by the three segments into three measurement beam parts 14 a,14 b, 14 c, each directed to one of the three detectors 12 a, 12 b, 12c, for instance photodiodes or other elements capable of measuringradiation intensity.

Since the structure of the segments of the arrays of segments isperiodic, each respective segment 13 a, 13 b, 13 c per array of segmentsdirects the part of the measurement beam received by that segmenttowards the associated detector 12 a, 12 b, and 12 c. Thus all firstsegments 13 a direct radiation of the measurement beam towards the firstdetector 12 a, second segments 13 b of the periodic structure towardsthe second detector 12 b, and third segments 13 c of the periodicstructure towards the third detector 12 c.

The measured radiation intensities are received by a processing unit 5,wherein on the basis of the radiation intensities received by thedifferent detectors, a height level of the substrate 2 can be deductedas now will be explained for an exemplary embodiment.

Again referring to FIG. 3, the periodic structure shown at the left sideof FIG. 3 is configured from rhombic shapes having a length L of about30 μm and a width W of about 4 μm. Due to the small NA of the imagingoptics used in the level sensor, the periodicity in the width directionof the projection grating is not resolved, while the periodicity in thelength direction L of the projection grating 7 is resolved. It isremarked that in an alternative embodiment the periodicity may also beresolved in the width direction.

At the right side of FIG. 3, the resulting projection image of thisperiodic structure on the substrate 2 is shown. The image of FIG. 3shows that the projection image has a periodicity in the lengthdirection of the projection grating 7. This image is reflected on thetop surface of the substrate 2 towards the detection unit 4. Theintensity distribution received by the detection grating 11 of thedetection unit 4 can be approximated by a sinusoidal intensitydistribution, wherein the intensity is dependent on the length directionvariable x and the shift s of the image caused by the substrate height.I=A+B cos(x+s)In this equation the pitch of the sinusoidal variation is chosen equalto 2π. The shift of the image s is determined by the substrate height.This shift s is determined to calculate the height of the substrate atthe respective measurement location 8, while taking into account theparameters A and B which are also unknown and variable.

FIG. 4 shows, for illustration, an aerial image AI of the sinusoidalintensity distribution above the three segments 13 a, 13 b, and 13 c.Each of the segments 13 a, 13 b, 13 c receives another part of theintensity distribution. Due to the different angles of the segments 13a, 13 b, 13 c each part of the intensity distribution is guided to therespective one of the detectors 12 a, 12 b, 12 c. The radiationintensities received by the detectors 12 a, 12 b, 12 c are guided to theprocessing unit 5 to determine the height level of the substrate at themeasurement location 8.

The radiation intensities D1, D2 and D3, received by each of thedetectors 12 a, 12 b, 12 c can be described by the following relations:

$D_{1} = {{\frac{2\pi}{3}A} + {B\left\lbrack {{\sin\left( {{- \frac{\pi}{3}} + s} \right)} - {\sin\left( {{–\pi} + s} \right)}} \right\rbrack}}$$D_{2} = {{\frac{2\pi}{3}A} + {B\left\lbrack {{\sin\left( {\frac{\pi}{3} + s} \right)} - {\sin\left( {{- \frac{\pi}{3}} + s} \right)}} \right\rbrack}}$$D_{3} = {{\frac{2\pi}{3}A} + {B\left\lbrack {{\sin\left( {\pi + s} \right)} - {\sin\left( {\frac{\pi}{3} + s} \right)}} \right\rbrack}}$From these three equations with 3 unknown variables, two quadraturesignals can be derived.

${B\;{\cos(s)}} = \frac{{2D_{2}} - D_{1} - D_{3}}{3\sqrt{3}}$${B\;{\sin(s)}} = \frac{D_{1} - D_{3}}{3}$These two quadrature signals allow the value s to be found for any valueof s so there are no linearity errors and no dead-zones with zerosensitivity for substrate height changes. As a result, the level sensor1 is suitable to determine the height of the substrate in a relativelylarge height range of more than +/−5 μm, or even +/−10 μm. Therefore,the need of closed-loop height control during measuring of the heightlevel may be obviated.

In an further embodiment, the detection grating may comprise four ormore segments for each period of the measurement beam. In the embodimentshown in FIG. 4, the segments 13 a, 13 b, 13 c each have the same length1 s. In an alternative embodiment, the segments may have differentlengths, as long as the length of the complete array of segmentscorresponds to a period of the image of the measurement beam projectedon the detection grating 11.

The angles of the top surfaces of the segments are about −15°, 0° and15° with respect to the main plane of the detection grating 11. Anyother suitable angle may be applied for each of the segments. Thedifference between the angles should be large enough to split themeasurement beam in three distinguishable beam parts that can bedirected to the three distinct detectors 12 a, 12 b, 12 c, so thatintensity differences between the three beam parts can be determined.

Since the measurement of a height level of the substrate 2 can bemeasured open-loop, and in a relatively large height range, the levelsensor 1 is suitable for measuring simultaneously the height level atmultiple measurement locations 8. To locate the measurement location 8of the level sensor 1 at different positions on the substrate 2,different methods may be applied.

In an embodiment, the substrate 2 may be moved along the level sensor 1in a scanning movement. Since no closed loop height control may berequired, this movement can be made at a constant speed therewithavoiding the need of accelerations of the substrate support supportingthe substrate 2 during determining a height level of a substrate. In afurther embodiment, the level sensor 1 may be moved over the substrate 2while the substrate is stationary. In a further embodiment, both thelevel sensor 1 and the substrate may be moved to obtain an optimum pathto move the measurement locations over the surface of the substrate.

In an embodiment, the level sensor may be provided with a movableradiation guiding device configured to direct the measurement beam atdifferent locations on the substrate without moving the complete levelsensor and/or substrate. With such a level sensor, at least the heightof a part of a substrate, for instance a line of measurement locations,may be measured without movement of the complete level sensor and/orsubstrate being required. In such an embodiment, less movement or lesscomplex movement of the substrate with respect to the level sensor maybe necessary to obtain a height map of a complete substrate.

FIG. 5 shows a further embodiment of a level sensor 101. In the levelsensor 101 of FIG. 5, the same or similar features of the embodiment ofFIG. 2 are indicated with the same reference numerals.

The level sensor 101 is configured to measure a height of a substrate atdifferent measurement locations 108 spaced on a line crossing the widthof the substrate 2. For each measurement location 108, the level sensor101 comprises the components of the level sensor shown in FIG. 2. Thuseach measurement location 108 is associated with a projection unitcomprising a radiation output 6 and a projection grating 7, and adetection unit comprising a detection grating 11 and a number ofdetectors (not shown). Furthermore, optical elements 9 and 10 areprovided to guide the measurement beam from the projection unit to themeasurement location and, after reflection on substrate 2, from themeasurement location to the detection unit.

In the depicted embodiment, for each measurement location 108, oneradiation output 6, one detection grating 11 and a set of three or moredetectors is provided. There is one projection grating 7 and one set ofoptical elements 9 and 10 for all the measurement locations. Theprojection grating 7 and the optical elements 9,10 extend for thisreason along the measurement locations 108.

In an alternative embodiment, one detection grating 11 may be providedto be used for all measurement locations 8. Also or alternatively, oneor more parts, for instance projection grating, detection grating, etc.may be provided for multiple, but not all, measurement locations 8.

The level sensor shown in FIG. 5 is configured to measure the height ofa substrate at twelve measurement locations 8. The full width of thesubstrate can be covered by this line of measurement locations 8. Whendesired more or less measurement locations 8 may be provided. Themeasurement locations 8 may also be arranged in other configurationsthan along one line as shown in the embodiment of FIG. 5.

The construction of the level sensor 1 as shown in FIG. 2 is inparticular suitable to be used in a multiple measurement location levelsensor since the level sensor is capable of measuring the height of thesubstrate 2 surface in a relatively large height range. Typically, thislarger height range for measurement is larger than normal variations inthickness and/or flatness of a substrate. This larger measurement rangemay obviate closed-loop height control. As a result, the level sensor ofFIG. 5 may be used to measure the substrate height simultaneously atmultiple measurement locations 8 without the requirement of correctingthe position of the substrate due to height differences between theheights of the substrate at the different measurement locations.

The multiple measurement locations 8 may be moved along the substrate toobtain height information of a large number of locations on thesubstrate 2. This movement may be obtained by movement of the levelsensor 101, movement of the substrate 2, and/or movement of a radiationguiding element of the level sensor 101, for instance movement of theoptical elements 9 and 10. A combination of these movements may be usedto obtain a height map of the substrate 2 to be used for correctionalmovements of the substrate during the actual lithographic process.

Tilt Measuring Device

FIG. 2 shows a level sensor 1 capable of measuring the height of asubstrate with relatively high accuracy. The level sensor may provide areliable measurement within a height range of +/−5 μm, or even +/−10 μmor possibly even larger. However, when the image of the projectiongrating becomes defocused relative to the detection grating, the levelsensor may become sensitive to tilt of the substrate about the x-axis.Although this effect is small, the tilt of the substrate 2 may bemeasured and taken into account when the height map of a substrate isdetermined. The correction of the determined height by taking the tiltof the substrate into account may improve the accuracy of the height mapeven further. An example of a level sensor capable of measuring the tiltof the substrate, will now be discussed.

FIG. 6 shows the level sensor of FIG. 2, further comprising a tiltmeasuring device 20. The tilt measuring device 20 is arranged to receiveat least part of the reflected measurement beam, and configured toprovide a tilt signal representative for a tilt of the substrate aboutthe x-axis (Rx) with respect to a nominal plane 21. The tilt measuringdevice 20 is connected to the processing unit 5 to feed the tilt signalto the processing unit 5 in order to correct, when needed, the heightlevel measured by the level sensor 1.

In the level sensor 1, the measurement beam reflected on the substrate 2is received by the optical element 10 which is a partially reflectivemirror which reflects the major part of the measurement beam, forexample 80% of the radiation intensity towards the detection unit 4.

The rest of the measurement beam is transmitted through the partiallyreflective mirror and is used to detect the tilt of the substrate in thetilt measuring device 20. This rest of the measurement beam is indicatedin FIG. 6 as tilt measurement beam 22.

The tilt measurement beam 22 is received by a beam splitting device 23.The beam splitting device 23 is configured to divide the tiltmeasurement beam in two parts, each part directed to one of two tiltdetectors 24, 25. The beam splitting device 23 comprises two wedgesplaced against each other. The part of the tilt measurement beamreceived by the lower wedge is guided to the tilt detector 24, and thepart of the tilt measurement beam received by the upper wedge is guidedto the tilt detector 25. The two wedges of the beam splitting device 23are spaced in a tilt sensitive direction.

Comparison of a ratio between relative amounts of radiation received bythe two tilt detectors 24, 25 is representative for the tilt of thesubstrate. For instance, when the substrate is tilted as indicated bythe arrow T in FIG. 6, the part of the tilt measurement beam 22 receivedby the upper wedge will increase, while the part received by the lowerwedge will decrease. The differences between the amounts of radiationreceived by the two tilt detectors 24, 25 can be normalized to determinesmall intensity differences between the radiation received by the tiltdetectors 24, 25. This comparison of the amount of radiation received bythe respective tilt detectors 24, 25 results in detection of small tiltvariation of the substrate 2.

The tilt measuring device of FIG. 6 is only an example of a tilt sensorthat can be used to measure the tilt of the substrate to correct theheight determined by the tilt measurement device 20. Any other tiltmeasuring device 20 capable of measuring the tilt of a substrate may beapplied. Generally it will be desirable to measure the tilt about anaxis perpendicular to the plane in which the measurement beam of thelevel sensor runs. In the shown embodiment, the measurement beam runs inthe y-z plane; thus the tilt to be measured is tilt of the substrateabout the x-axis (Rx).

The tilt measuring device 20 as shown in FIG. 6 can be constructed to bevery compact, and is therefore suitable to be applied in a level sensorsystem for multiple measurement locations as shown in FIG. 5. In such alevel sensor, for each measurement location of multiple measurementlocations, a tilt measuring device 20 may be provided to measure foreach measurement location 8 a tilt variable of the substrate so that foreach measurement location 8 the height determined by the level sensor101 may be corrected for a tilt of the substrate 2.

Measurement Beam Delivery System

In FIG. 5, a top view of a level sensor 101 is shown configured tomeasure a height level of a substrate 2 at multiple measurementlocations 8. This level sensor 101 provides the possibility to measurethe height of the substrate 2 simultaneously at the differentmeasurement locations 8. This level sensor further provides thepossibility for open loop scanning of the measurement locations alongthe surface of the substrate 2. When desired, the tilt of the substrate2 may be measured by one or more tilt measuring devices to correct theheight measured by the level sensor 1 to correct the measured height forany effect due to a tilt of the substrate 2.

Although this level sensor provides possibilities for fast and accuratedetermination of a height map of a substrate 2, the level sensor asshown in FIG. 5 may require substantial space above the substrate 2.Such space may not always be available above the substrate in alithographic apparatus.

To use the space available in a lithographic apparatus more efficiently,FIG. 7 shows an alternative beam delivery method to provide measurementbeams at multiple measurement locations at the substrate which multiplemeasurement locations 8 are arranged on a line. The substrate 2 isarranged in a measurement position, i.e. at least one of the measurementlocations 8 of the level sensor is located somewhere on the substrate 2so that height measurement of the substrate 2 can be carried out.

For each measurement location 8, a radiation output 6, a projectiongrating 7, a detection grating 11 and three detectors (not shown) areprovided. These components of the level sensor as such correspond to thecomponents used in the level sensor shown in FIGS. 2 and 5. However, theradiation output 6 and the projection grating 7 are provided at one sidenext to the substrate 2, while the detection grating 11 and thedetectors are provided at the other side next to the substrate 2. Thedifferent components may be located at different heights with respect tothe substrate 2, but in the shown embodiment all components are arrangedat a height level higher than the substrate 2.

An advantage of this arrangement is that no components are arrangedabove the substrate 2, and all components at one side of the substrate 2are arranged relative closely to each other resulting in a relativelysmall space required for all components in a lithographic apparatus.

It is remarked that the term “next to” is used to indicate a locationthat when projected perpendicularly from the main plane of the substratelies outside the substrate. The terms “above” or “below” indicate alocation that when projected perpendicularly from the main plane of thesubstrate lies within the substrate.

FIGS. 8, 9 and 10 show a further embodiment in which the space requiredfor the measurement beams of the level sensor can be further reduced. Inthe embodiment of FIGS. 8, 9 and 10, reflection bars 30, 31 are providedabove the substrate 2. The reflection bars 30, 31 are each provided withan oblique reflective surface 30 a, 31 a (see FIG. 9 which shows a viewalong line A-A in FIG. 8).

The angles of the oblique surfaces 30 a, 31 a are desirably equal withrespect to the horizontal plane and are directed towards the surface ofthe substrate. The angles are chosen such that a measurement beamreceived by the reflection bar 30 in a substantially horizontal plane isreflected towards a measurement location 8 on the substrate 2. At themeasurement location 8 the measurement beam is reflected back towardsthe reflection bar 31. The reflected beam is received on the reflectionbar 31 at substantially the same height as where it was reflected on thereflection bar 30. From the reflection bar 31 the measurement beam isguided in a substantially horizontal direction to the detection unit 4.

In the detection unit 4 further optical elements may be provided todetect signals representative for the height of the substrate 2. Thesesignals may be used to determine the height of the substrate 2 at themeasurement location in a processing unit which may be arranged in thedetection unit 4 or at any other suitable location. The components ofthe level sensor may correspond to the components of the above describedembodiments, but any other suitable level sensor device may also beused.

The sensor components are depicted by a projection unit 3 and adetection unit 4. The projection unit 4 is configured to providemultiple measurement beams to measure a height level at differentmeasurement locations 8 on the substrate 2. All measurement beams areprovided at substantially the same height and directed to the reflectionbar 30. All reflected measurement beams are received at substantiallythe same height by the reflection bar 31, and directed towards thedetection unit 4.

An advantage of the measurement beam delivery device of the embodimentshown in FIGS. 8, 9, and 10 is that the measurement beam can be kept ata relatively low height level with respect to the substrate 2, while atthe same time the projection unit 3 and the detection unit 4 arearranged next to the substrate where more space may be available for theprovision of these units 3, 4.

In the embodiment shown in FIGS. 8, 9, and 10, only three measurementlocations 8 are shown. In practice, many more measurement locations 8,for instance over 50 measurement locations, may be located at a line orin any other suitable configuration. Furthermore, the substrate 2 may bemoved to move the measurement locations 8 over the substrate surface todetermine the height level of the substrate over the complete surface ofthe substrate to determine a height map of the substrate. The movementmay be realized by movements of the level sensor and/or movements of thesubstrate, or by any other suitable method.

It is remarked that the projection unit 3 and detection unit 4, whenusing the reflection bars 30, 31 may also be located partially orcompletely above the substrate in the measurement location.

Further, it is remarked that the projection unit 3 and/or detection unit4 of the embodiments of FIGS. 7, 8, 9, and 10 may comprise a compensatorto compensate, when necessary, for any difference in measurement beampath length between the different measurement beams.

It is also remarked that the measurement beams of the level sensor maybe collimated beams. An advantage of using collimated beams (e.g. laserbeams) is an increased feasibility of the level sensor.

In an embodiment, there is provided a level sensor configured to measurea height level of a substrate arranged in a measurement position, thelevel sensor comprising: a projection unit to project multiplemeasurement beams on multiple measurement locations on the substrate; adetection unit to receive the measurement beams after reflection on thesubstrate; and a processing unit to calculate a height level on thebasis of the reflected measurement beams received by the detection unit,wherein the projection unit and the detection unit are arranged next tothe substrate, when the substrate is arranged in the measurementposition.

In an embodiment, the measurement locations are arranged on a line. Inan embodiment, in a projection on a main plane of the substrate, themeasurement beams are non-perpendicular to the line on which themeasurement locations are arranged. In an embodiment, the projection themeasurement beams run non-parallel to the line on which the measurementlocations are arranged. In an embodiment, the measurement beams arecollimated beams. In an embodiment, the level sensor comprises a firstreflection bar arranged to receive the multiple measurement beams fromthe projection unit, and to direct the measurement beams to thesubstrate, and a second reflection bar arranged to receive the reflectedmultiple measurement beams from the projection unit, and to direct themeasurement beams to the detection unit. In an embodiment, the firstreflection bar is configured to receive the multiple measurement beamsfrom the projection unit in a plane substantially parallel to a mainplane of the substrate. In an embodiment, the first reflection barcomprises an oblique reflective surface to reflect the multiplemeasurement beams towards the substrate, and wherein the secondreflection bar comprises an oblique reflective surface to reflect thereflected multiple measurement beams towards the detection unit. In anembodiment, the second reflection bar is configured to direct themultiple measurement beams to the detection unit from a planesubstantially parallel to a main plane of the substrate. In anembodiment, the first and second reflection bars are point symmetric. Inan embodiment, the first and the second reflection bars are arrangedabove the substrate of which a height map is to be determined. In anembodiment, the projection and/or detection unit comprises a compensatorto compensate for difference in beam path length between differentmeasurement beams. In an embodiment, the projection unit comprises aradiation output to provide a measurement beam, and a projection gratingarranged to receive the measurement beam and to impart the measurementbeam with a periodic radiation intensity, and wherein the detection unitcomprises a detection grating arranged to receive the reflectedmeasurement beam, and a detector arranged to receive the measurementbeam. In an embodiment, the level sensor further comprises a tiltmeasuring device arranged to receive at least part of the reflectedmeasurement beam, and configured to provide a tilt signal representativeof a tilt of the substrate with respect to a nominal plane.

In an embodiment, there is provide a lithographic apparatus comprising:an illumination system configured to condition a radiation beam; asupport constructed to support a patterning device, the patterningdevice being capable of imparting the radiation beam with a pattern inits cross-section to form a patterned radiation beam; a substrate tableconstructed to hold a substrate; a projection system configured toproject the patterned radiation beam onto a target portion of thesubstrate; and a level sensor configured to measure a height level of asubstrate on the substrate table arranged in a measurement position,comprising: a projection unit to project multiple measurement beams onmultiple measurement locations on the substrate, a detection unit toreceive the measurement beam after reflection on the substrate, and aprocessing unit to calculate a height level on the basis of thereflected measurement beam received by the detection unit, wherein theprojection unit and the detection unit are arranged next to thesubstrate, when the substrate is arranged in the measurement position.

In an embodiment, there is provided a method to determine a height mapof a substrate, the method comprising: measuring a height of a substrateat a measurement location by projecting a measurement beam on thesubstrate using a projection unit, receiving the reflected beam from thesubstrate using a detection unit, and determining a height level basedon the reflected beam received by the detection unit, wherein theprojection unit and the detection unit are arranged next to thesubstrate, when the substrate is arranged in a measurement position;repeating the measuring for multiple measurement locations on thesubstrate; and determining a height map of the substrate on the basis ofthe height levels.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin-film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion”, respectively. The substrate referred toherein may be processed, before or after exposure, in for example atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist), a metrology tool and/or an inspectiontool. Where applicable, the disclosure herein may be applied to such andother substrate processing tools. Further, the substrate may beprocessed more than once, for example in order to create a multi-layerIC, so that the term substrate used herein may also refer to a substratethat already contains multiple processed layers.

Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography, itwill be appreciated that the invention may be used in otherapplications, for example imprint lithography, and where the contextallows, is not limited to optical lithography. In imprint lithography atopography in a patterning device defines the pattern created on asubstrate. The topography of the patterning device may be pressed into alayer of resist supplied to the substrate whereupon the resist is curedby applying electromagnetic radiation, heat, pressure or a combinationthereof. The patterning device is moved out of the resist leaving apattern in it after the resist is cured.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of or about 365, 248, 193, 157 or 126 nm) andextreme ultra-violet (EUV) radiation (e.g. having a wavelength in therange of 5-20 nm), as well as particle beams, such as ion beams orelectron beams.

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic and electrostaticoptical components.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. For example, the invention may take the form of acomputer program containing one or more sequences of machine-readableinstructions describing a method as disclosed above, or a data storagemedium (e.g. semiconductor memory, magnetic or optical disk) having sucha computer program stored therein.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made to the invention as described without departing from thescope of the claims set out below.

What is claimed is:
 1. A level sensor configured to measure a heightlevel of a substrate arranged in a measurement position, the levelsensor comprising: a projection unit to project multiple measurementbeams on multiple measurement locations on the substrate; a detectionunit to receive the measurement beams after reflection on the substrate;a processing unit to calculate a height level on the basis of thereflected measurement beams received by the detection unit; a firstreflection bar arranged to receive the measurement beams from theprojection unit, and to direct the measurement beams to the substrate,the first reflection bar comprising an oblique first reflective surfaceonto which the measurement beams are incident and to reflect themeasurement beams towards the substrate; and a second reflection bararranged to receive the reflected measurement beams from the substrate,and to direct the reflected measurement beams to the detection unit, thesecond reflection bar comprising an oblique second reflective surfaceonto which the reflected measurement beams are incident and to reflectthe reflected measurement beams towards the detection unit, wherein thefirst and second reflective surfaces face toward each other and towardthe substrate.
 2. The level sensor of claim 1, wherein the measurementlocations are arranged on a line.
 3. The level sensor of claim 2,wherein in a projection on a main plane of the substrate, themeasurement beams are non-perpendicular to the line on which themeasurement locations are arranged.
 4. The level sensor of claim 3,wherein in the projection the measurement beams run non-parallel to theline on which the measurement locations are arranged.
 5. The levelsensor of claim 1, wherein the measurement beams are collimated beams.6. The level sensor of claim 1, wherein the first reflection bar isconfigured to receive the multiple measurement beams from the projectionunit in a plane substantially parallel to a main plane of the substrate.7. The level sensor of claim 1, wherein the second reflection bar isconfigured to direct the multiple measurement beams to the detectionunit from a plane substantially parallel to a main plane of thesubstrate.
 8. The level sensor of claim 7, wherein the first and secondreflection bars are point symmetric.
 9. The level sensor of claim 7,wherein the first and the second reflection bars are arranged above thesubstrate of which a height map is to be determined.
 10. The levelsensor of claim 1, wherein the projection and/or detection unitcomprises a compensator to compensate for difference in beam path lengthbetween different measurement beams.
 11. The level sensor of claim 1,wherein the projection unit comprises a radiation output to provide ameasurement beam, and a projection grating arranged to receive themeasurement beam and to impart the measurement beam with a periodicradiation intensity, and wherein the detection unit comprises adetection grating arranged to receive the reflected measurement beam,and a detector arranged to receive the measurement beam.
 12. The levelsensor of claim 1, further comprising a tilt measuring device arrangedto receive at least part of the reflected measurement beam, andconfigured to provide a tilt signal representative of a tilt of thesubstrate with respect to a nominal plane.
 13. The level sensor of claim1, wherein the projection unit and the detection unit are arranged nextto the substrate, when the substrate is arranged in the measurementposition.
 14. A lithographic apparatus comprising: a support constructedto support a patterning device, the patterning device being capable ofproviding a radiation beam with a pattern in its cross-section; asubstrate table constructed to hold a substrate; a projection systemconfigured to project the patterned radiation beam onto a target portionof the substrate; and a level sensor configured to measure a heightlevel of a substrate on the substrate table arranged in a measurementposition, comprising: a projection unit to project multiple measurementbeams on multiple measurement locations on the substrate, a firstreflection bar arranged to receive the measurement beams from theprojection unit and to direct the measurement beams to the substrate, adetection unit to receive the measurement beams after reflection on thesubstrate, a second reflection bar arranged to receive the reflectedmeasurement beams from the substrate, and to direct the reflectedmeasurement beams to the detection unit, wherein the length of the firstand second reflection bars combine to be longer than the width of thesubstrate, a processing unit to calculate a height level on the basis ofthe reflected measurement beams received by the detection unit.
 15. Theapparatus of claim 14, wherein the projection unit and the detectionunit are arranged next to the substrate, when the substrate is arrangedin all measurement positions of the substrate.
 16. The apparatus ofclaim 14, wherein the first reflection bar comprises an oblique firstreflective surface onto which the measurement beams are incident and toreflect the measurement beams towards the substrate, the secondreflection bar comprises an oblique second reflective surface onto whichthe reflected measurement beams are incident and to reflect thereflected measurement beams towards the detection unit, and the firstand second reflective surfaces face toward each other and toward thesubstrate.
 17. The apparatus of claim 14, wherein the projection and/ordetection unit comprises a compensator to compensate for difference inbeam path length between different measurement beams.
 18. A method todetermine a height map of a substrate, the method comprising: measuringa height of a substrate at a measurement location by projecting ameasurement beam on the substrate using a projection unit and an obliquefirst reflective surface onto which the measurement beam is incident andto reflect the measurement beam towards the substrate, receiving thereflected beam from the substrate using a detection unit and an obliquesecond reflective surface onto which the reflected beam is incident andto reflect the reflected beam towards the detection unit, wherein thefirst and second reflective surfaces face toward each other and towardthe substrate, and determining a height level based on the reflectedbeam received by the detection unit; repeating the measuring formultiple measurement locations on the substrate; and determining aheight map of the substrate on the basis of the height levels.
 19. Themethod of claim 18, wherein the projection unit and the detection unitare arranged next to the substrate, when the substrate is arranged in ameasurement position.
 20. The method of claim 18, wherein the first andsecond reflection bars extend at least partially above the substrate andthe length of the first and second reflection bars combine to be longerthan the width of the substrate.